With the development of communications technologies, PON (Passive Optical Network, passive optical network) systems have already been widely applied in VoIP (Voice over Internet Protocol), WiFi (Wireless Fidelity, wireless fidelity), Ethernet, and other fields of fiber to the home and mobile backhaul. Referring to FIG. 1, a PON system mainly includes a central office device OLT (Optical Line Termination, optical line termination), a remote device ONU (Optical Network Unit, optical network unit), and an ODN (Optical Distribution Network, optical distribution network) located between the OLT and the ONU. The ODN mainly includes such components as an optical fiber and an optical splitter.
In a VoIP application, in order to improve the quality of a voice system, a function of voice synchronization needs to be implemented, that is, clock information of a voice terminal device comes from a network side and is synchronized with a network side clock source, so as to implement clock synchronization between interconnected terminal devices in a voice network. Mobile backhaul also has a strict requirement on clock synchronization, requiring that an Ethernet port of an ONU has a clock synchronization function of Ethernet synchronization, that is, a working clock of the Ethernet port comes from a PON network side and is synchronized with a network side clock source.
Recovery of clock information of the network side is key to implementation of the foregoing feature of clock synchronization. The clock information needs to be extracted and recovered from a data stream delivered by the network side. However, data delivered by the network side may be interrupted or may deteriorate due to various reasons, which may result in loss of data and related clock information. In this case, how to ensure that a recovered clock as a clock source for clock synchronization is output and has a stable quality is a key issue for a system.